STA-21 is a natural antibiotic that acts as a selective STAT3 inhibitor. It inhibits STAT3 by preventing its dimerization and DNA binding when used at concentrations of 20-30 μM. It blocks the growth and survival of cancer cells that express constitutively active STAT3 (e.g., DU145 cells, IC₅₀ = 12.2 μM). It reciprocally regulates Th17 and Treg cells, and inhibits osteoclast formation.